Atomic layer deposition (ALD) processes are useful for the creation of thin films, as described by M. Ritala and M. Leskela in “Atomic Layer Deposition” in Handbook of Thin Film Materials, H. S. Nalwa, Editor, Academic Press, San Diego, 2001, Volume 1, Chapter 2. Such films, especially metal and metal oxide films, are critical components in the manufacture of electronic circuits and devices.
In an ALD process for depositing copper films, a copper precursor and a reducing agent are alternatively introduced into a reaction chamber. After the copper precursor is introduced into the reaction chamber and allowed to adsorb onto a substrate, the excess (unadsorbed) precursor vapor is pumped or purged from the chamber. The removal of excess precursor vapor is followed by introduction of a reducing agent that reacts with the copper precursor on the substrate surface to form copper metal and a free form of the ligand. This cycle can be repeated if needed to achieve the desired film thickness.
The ALD process differs from chemical vapor deposition (CVD) in the decomposition chemistry of the metal complex. In a CVD process, the complex undergoes pyrolytic decomposition on contact with the surface to give the desired film. In an ALD process, the complex is not completely decomposed to metal on contact with the surface. Rather, formation of the metal film takes place on introduction of a second reagent, which reacts with the deposited metal complex. In the preparation of a copper film from a copper(I) complex, the second reagent is a reducing agent. Advantages of an ALD process include the ability to control the film thickness and improved conformality of coverage because of the self-limiting adsorption of the precursor to the substrate surface in the first step of the process.
The ligands used in the ALD processes are desirably stable with respect to decomposition and should be able to desorb from the complex in a metal-free form. Following reduction of the copper, the ligand is liberated and must be removed from the surface to prevent its incorporation into the metal layer being formed.
US 2003/0135061 discloses a dimeric copper(I) precursor which can be used to deposit metal or metal-containing films on a substrate under ALD or CVD conditions.